Steel for submarine hulls with improved weldability

ABSTRACT

The invention relates to a steel for the production of submarine hulls having the chemical composition:
 
0.030%≦C&lt;0.080%
 
0.040%≦Si≦0.48%
 
0.1%≦Mn≦1.4%
 
2%≦Ni≦4%
 
Cr≦0.3%
 
0.30%≦Mo+W/2+3(V+Nb/2+Ta/4)≦0.89%
 
Mo≧0.15%
 
V+Nb/2+Ta/4≦0.004%
 
Nb≦0.004%
 
Cu≦0.45%
 
Al≦0.1%
 
Ti≦0.04%
 
N≦0.0300%
 
the balance being iron and impurities resulting from the production operation, boron being an impurity whose content is less than 0.0005%, and P+S≦0.015%, the chemical composition complying with the condition:
 
410≦540×C 0.25 +245[Mo+W/2+3(V+Nb/2+Ta/4)] 0.30 ≦460.

The present invention relates to a steel for the production of submarinehulls which are constituted by rolled or forged steel components whichare assembled by means of welding.

BACKGROUND OF THE INVENTION

In order to be able to immerse in deep water without making the vesselexcessively heavy, hulls of submarines are generally constituted bysheets of steel having a thickness of between 40 and 50 mm, andoptionally forged components which have a thickness of between 100 and150 mm and which are constituted by a very high strength steel having avery good impact resistance even at low temperatures, so as to provide agood degree of reliability even in the case of intense dynamic loads,and which can be relatively easily welded so as to allow assemblies of ahigh quality to be produced.

The steels which are conventionally used are steels of the seriesreferred to as 60 or 80 HLES, whose chemical composition comprisesapproximately 0.10% of carbon, from 2 to 4% of nickel, from 0.2 to 0.4%of silicon, molybdenum and vanadium at contents such that Mo+3V isbetween 0.3 and 0.5%, between 0.8 and 1.2% of Mn, between 0.1 and 0.5%of Cr, the balance being iron, impurities and optionally low quantitiesof deoxidation elements. These steels are used to produce componentssuch as sheets or forged components which are quenched and tempered soas to have a tempered structure which is on the whole martensitic, thatis to say, which contains more than 90% of martensite and of which theyield strength is between 550 and 650 MPa, the tensile strength isbetween 600 and 750 MPa, the elongation at break is between 15 and 20%,the Charpy toughness K_(cv) is greater than 80 J at −80° C.

Components which are produced from these steels are assembled by meansof welding with preheating to a temperature in the order of at least150° C. in order to prevent problems of cracking in the cold state.

These welding conditions are required in particular because the weldseams which are produced are weld seams which are very extensivelyflanged and which can produce stresses of almost 80% of the elasticlimit, and because the weld seams are carried out on sites in which thetemperature may drop to a level in the region of 0° C.

The need for carrying out a preheating operation at a high temperatureis a disadvantage which makes it difficult to weld components ofsubmarine hulls. Therefore, it is desirable to be able to have a steelwhich allows weld seams to be produced under less severe conditions,that is to say, without pre-heating, or at least by carrying out only abaking operation of the sheets which does not exceed 100° C., orpreferably, 50° C., in spite of the very extensive flanges of the weldseams and in spite of the relatively low outside temperatures of thesite.

It has been proposed, in particular in patent application WO93/24269,that the welding conditions be improved for submarine hulls which areproduced from a steel of the 60 or 80 HLES type, by using weldingelectrodes which are different from the electrodes defined by theconstruction standards which are applied in this field, these weldingelectrodes leading to low-carbon bainitic structures (LCBS).

However, this technique has disadvantages since the reduction of therisk of cracking which is thus obtained in the region of the depositedmetal nonetheless does not overcome the problem of the risk of crackingbrought about by the welding operation in the base metal itself, in theregion of the heat affected zone.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome these disadvantagesby providing a weldable steel with a high yield strength for producingsubmarine hulls by assembling components, by means of welding, which areconstituted by thick sheets or forged components and which have a yieldstrength of between 480 and 620 MPa, a toughness measured in terms of aCharpy V K_(v) greater than 50 J at −60° C., preferably greater than 50J at −85° C., and for which there is a reduction in the risks ofoccurrences of cracking in the base metal, brought about by a weldingoperation in the region of the heat affected zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows “Si % normal” and “Si % reduced” of the Examples setforth in Tables 1 and 2.

DESCRIPTION OF THE INVENTION

To this end, the invention relates to a steel for producing submarinehulls, characterised in that the chemical composition thereof comprises,in % by weight:0.030%≦C<0.080%0.040%≦Si≦0.48%0.1%≦Mn≦1.4%2%≦Ni≦4%Cr≦0.3%0.30%≦Mo+W/2+3(V+Nb/2+Ta/4)≦0.89%Mo≧0.15%V+Nb/2+Ta/4≦0.004%Nb≦0.004%Cu≦0.45%Al≦0.1%Ti≦0.04%N≦0.0300%the balance being iron and impurities resulting from the productionoperation, boron being an impurity whose content is less than 0.0005%,and P+S≦0.015%, the chemical composition complying with the condition:410≦540×C^(0.25)+245[Mo+W/2+3(V+Nb/2+Ta/4)]^(0.30)≦−460the steel having a structure which is substantially martensitic or lowerbainitic or which is substantially constituted by a mixture of these twostructures, comprising at least 90% of martensite or lower bainite or amixture of these two structures, a maximum of 5% of residual austenite,a maximum of 5% of ferrite, with a yield strength of between 480 MPa and620 MPa and a Charpy toughness V, K_(cv), greater than 50 J at −60° C.

Preferably, the chemical composition is such that one or more of thefollowing conditions is complied with:Si≦0.19%Mn≦1%W≧0.11%2.5%≦Ni≦3.5%Cr≦0.2% and preferably Cr≦0.09%425≦540×C^(0.25)+245[Mo+W/2+3(V+Nb/2+Ta/4)]^(0.30)≦450Ni≧2.7%Mo≦0.75%C≦0.055%

The invention also relates to a submarine hull component having athickness of between 15 mm and 150 mm, of forged or rolled steel whichhas been quenched and tempered according to the invention and asubmarine hull comprising components according to the invention whichare assembled by means of welding.

The invention finally relates to the use of the steel according to theinvention for producing a submarine hull comprising components whichhave thicknesses of between 15 mm and 150 mm and which are assembled bymeans of welding, the welding being able to be carried out on componentswhich have not been pre-heated or which have been preheated to atemperature which does not exceed 25° C.

The invention will now be described in a more precise but non-limitingmanner with reference to the single FIGURE which illustrates the path ofthe minimum temperature of non-cracking during welding as a function ofthe content of highly carbide-producing elements, and illustrated bymeans of examples.

The inventors found in a novel and unexpected manner that it waspossible to produce, for submarine hulls which have features inaccordance with the features generally required for producingsubmarines, components which can be more readily welded than componentsfor submarines produced from known steel. To this end, it was necessaryto use a steel whose composition had been modified compared with that ofknown steels, on the one hand, by substantially lowering the contents interms of carbon and, on the other hand, by increasing the contents interms of highly carbide-producing elements, that is to say, elementswhich are capable of precipitating in the form of fine and dispersedcarbides which harden but do not embrittle, this steel having to have astructure which is substantially martensitic, that is to say, whichcomprises more than 90% of martensite, the balance being constituted byless than 5% of residual austenite and less than 5% of ferrite. However,it has been found that the martensite component may, without excessivelyimpairing the toughness, be completely or partially replaced withbainite of the lower type, that is to say, bainite in the form of lathswhich in this respect have a micrographic appearance which is similar tomartensite.

The chemical composition of the steel which has been modified in thismanner comprises in % by weight:

-   -   more than 0.03% of carbon and preferably more than 0.035%, but        less than 0.080% and preferably less than 0.060% and even more        preferably less than 0.055%, in order to allow, on the one hand,        the formation of hardening carbides during tempering but without        impairing the toughness of the base metal and in particular the        toughness in the heat affected zone during the welding        operation; the content in terms of carbon being limited in        particular in order to reduce the deformations linked to the        martensitic transformation in the zone affected by the        temperature in the welding zone, which is necessary in order to        limit the effects of flanging and therefore reduce the        susceptibility of the metal to cracking during the welding        operation,    -   from 0.04% to 0.48% of silicon in order to deoxidise the bath of        liquid steel. Preferably, however, the silicon content will be        reduced and will remain less than 0.29%, preferably less than        0.25% and, even more preferably, less than 0.19% so as to        improve the thermal conductivity of the steel, which will have        the effect of reducing the thermal gradients during the welding        operation and thus reducing the resulting mechanical        constraints, which reduces the susceptibility of the steel to        cracking during the production of flanged weld seams,    -   up to 1.4% of manganese in order to improve the quenchability        but without forming excessively large segregated strips. Since        the steel also contains other quenching elements, manganese is        not strictly indispensable and the content thereof may be        limited to 1.2%, and more preferably to 1.0%; it can also be        present in trace levels. However, in order in particular to        facilitate the production of the steel, the manganese content        will preferably be at least equal to 0.2%, even 0.6%,    -   at least 2.1% and more preferably 2.5% and even more preferably        2.7% of nickel in order to improve the quenchability, which is        necessary in order to ensure that the desired type of        microstructure is obtained, that is to say, a structure which is        substantially constituted by martensite or lower bainite or a        mixture of these two structures. The nickel content may be up to        5%, but in practice and taking into account the cost of this        element, the content will preferably be less than 4% and even        more preferably less than 3.5%,    -   less than 0.3% and preferably less than 0.15% and even more        preferably less than 0.09% of chromium. This carbide-producing        element is not desirable. It is capable of forming relatively        solid carbides which do not have a particularly advantageous        effect on the properties of the steel according to the        invention. Consequently, if it were too high a quantity, it        would consume carbon which would then no longer be available to        form hardening carbides with other highly carbide-producing        elements which form hardening carbides which are fine and        dispersed. Chromium is therefore considered to be a residue        resulting from the production operation. Compliance with the        limitation relating to the content of this element means that        the steel has to be produced from raw materials which are        selected with care. These precautions are important more        particularly when the raw materials are constituted principally        by scrap iron which is generally the case for this type of        steel,    -   highly carbide-producing elements which form precipitated        carbides which are fine and hardening. These elements are        molybdenum and tungsten, on the one hand, vanadium, niobium and        tantalum, on the other hand. For these elements, the total by        weight [Mo+W/2+3(V+Nb/2+Ta/4)] must be at least 0.30%, and        preferably 0.35%, and even more preferably 0.4%. This total must        not be too high in order to limit the unfavourable effects on        the toughness and homogeneity of the metal, which would result        from contents beyond that which is necessary for the desired        hardening. Consequently, the total by weight of highly        carbide-producing element remains less than 0.89%, and        preferably less than 0.69%, and more preferably less than 0.59%.        Furthermore, molybdenum and tungsten are preferable since        vanadium, niobium and tantalum have a significant embrittling        effect. Therefore, the total by weight V+Nb/2+Ta/4 remains less        than 0.004%, and since niobium is more harmful to the toughness        than vanadium, the content thereof is limited to 0.004% and the        elements vanadium, niobium and tantalum are preferably present        in trace levels. Conversely, the content in terms of molybdenum        will be a minimum of 0.15% and more preferably 0.30% and even        more preferably 0.45%. Molybdenum may be preferred to tungsten        since it is more commonly used and is generally more economical        than tungsten. However, tungsten has the advantage of reducing        the formation of segregated zones which have unfavourable        effects on the toughness of the metal, and it is thus still        preferable to have a content of tungsten greater than 0.11%,    -   less than 0.45%, and more preferably less than 0.25% of copper        so as not to impair the forgeability and to promote the        suitability of the sheets for subsequent shaping,    -   up to 0.10% of aluminium and preferably less than 0.040%, but        preferably more than 0.004%, and more preferably more than        0.010%, in order to deoxidise the steel and form aluminium        nitrides which allow the growth of the grain to be controlled        during the thermal processing operation,    -   the content of nitrogen is preferably between 0.0010% and        0.0150% in order to facilitate the formation of aluminium        nitrides which allow the growth of the grain to be controlled;        the content in terms of nitrogen may exceed 0.0150%, but it is        desirable for this content not to exceed 0.0300% and preferably        not to exceed 0.0200% so as not to impair the suitability for        shaping the products in a cold or tepid state,    -   optionally up to 0.04% of titanium, an element which has a        comparable effect to that of aluminium. However, since titanium        has a tendency to form precipitates which have a very        embrittling effect, it is preferable to limit the content of        this element to trace levels.

The balance of the composition is constituted by iron and impuritiesresulting from the production operation. Amongst these impurities, boronmust remain at trace levels, that is to say, at contents of less than0.0005%. This is to prevent the embrittling effect of this element andits compounds in the form of nitrides and carbides. Although boron is anelement which is commonly used in order to enhance the quenchability ofsteels having a high elastic limit, the very high level of impactresistance desired in this instance leads to the use of the contributionof boron being avoided. Also amongst the impurities, phosphorus andsulphur must be limited to contents such that the total P+S remains lessthan 0.015%, and preferably less than 0.012%, and even more preferablyless than or equal to 0.009% in order not to impair the toughness of thesteel. This restriction requires the steel to be produced withparticularly strict precautions being taken. Nowadays, a person skilledin the art, who must comply with these restrictions, is aware of how toproceed.

Furthermore, in order to obtain adequate mechanical characteristics andin particular the yield strength and tensile strength, the chemicalcomposition of the steel must be such that the quantityR=540×C^(0.25)+245[Mo=W/2=3(V=Nb/2+Ta/4)]^(0.30)is between 410 and 460 and preferably between 425 and 450.

In order to produce sheets or components which are intended forproducing submarines, it is possible to proceed as indicated below.

First of all, the steel is produced in known manner, for example, in anelectric oven, taking all the necessary precautions known to the personskilled in the art in order to comply with the purity restrictions ofthe steel indicated above, the steel is then poured in the form of barsor slabs depending on the type of components it is desirable to produce.The bars or slabs are then formed by means of plastic deformation in thehot state, that is to say, by means of rolling or forging, by reheatingthem to a temperature such that the beginning of the transformation atheat is carried out at a temperature greater than 1000° C., andpreferably greater than 1050° C., and even more preferably greater than1100° C., in order to limit surface defects. However, the reheatingtemperature must preferably remain less than 1260° C. and morepreferably less than 1220° C., in order in particular to limit excessivegrowth of the grain at this stage. After the operation for shaping bymeans of plastic deformation in the hot state, the components producedare subjected to a thermal processing operation relating to the qualitycomprising a quenching operation either starting from the forming heator preferably, after reaustenitisation at a temperature which is atleast equal to AC3 and generally between approximately 860 and 950° C.The cooling can be achieved in accordance with all known quenchingmeans, such as those using air, oil or water, in accordance with thesolidity of the components in question in order to obtain asubstantially martensitic microstructure after quenching. The personskilled in the art knows how to select, on a case by case basis, themost suitable quenching means.

The quenching is followed by at least one tempering operation which ispreferably carried out at a temperature of between 550° C. and 670° C.

Using this method, it is possible to obtain sheets or forged componentswhose mechanical characteristics over the entire thickness are inaccordance with that desired for the production of submarines, that isto say, a yield strength of between 480 MPa and 620 MPa and preferablybetween 500 MPA and 600 MPa, and a Charpy toughness K_(cv) greater than50 J at −60° C.

The effect of the chemical composition on the suitability for welding isillustrated by the examples whose analyses are set out in tables 1 and2. For the examples of the tables, niobium, tantalum and titanium are attrace levels so that the values of Mo+W/2+3V indicated in the table areequal to the quantities Mo+3(V+Nb/2+Ta/4); boron is at trace levels witha content of less than 0.0005%; aluminium is between 0.015% and 0.025%.In table 1, the total P+S is expressed in 10⁻³%. In table 2, the totalP+S remains less than 0.015%.

Examples 1 and 2 are in accordance with the prior art and examples 3 to9 and 6a to 9a are in accordance with the invention. Example 10 is givenby way of comparison.

TABLE 1 n° C Si Mn Ni Cr Mo 1 0.092 0.300 0.550 3.000 0.060 0.270 20.083 0.400 0.800 3.000 0.070 0.160 3 0.069 0.320 0.900 3.200 0.0800.320 4 0.062 0.350 0.920 3.100 0.090 0.320 5 0.055 0.320 0.510 3.2500.130 0.210 6 0.049 0.280 0.920 3.300 0.095 0.395 7 0.043 0.300 0.5802.950 0.085 0.470 8 0.035 0.290 0.980 3.100 0.095 0.590 9 0.030 0.3201.010 2.950 0.120 0.555 10  0.023 0.300 0.960 3.150 0.090 1.015 (Mo +W/2) + n° W V Cu P + S 3 V Re Mpa Kv-85° C. (J) 1 tr 0.003 0.250 9 0.279575 205 2 0.115 0.024 0.150 8 0.291 570 135 3 tr 0.003 0.150 11  0.329565 210 4 tr 0.001 0.180 9 0.323 545 230 5 0.240 0.002 0.250 7 0.336 560225 6 tr 0.003 0.160 8 0.404 570 260 7 tr 0.001 0.180 7 0.473 570 260 8tr 0.002 0.200 9 0.596 565 275 9 0.360 0.003 0.210 6 0.744 570 280 10 tr 0.002 0.210 8 1.021 570 295

TABLE 2 Kv- Re 85° C. n° C Si Mn Ni Cr Mo W V Cu (Mo + W/2) + 3 V MPa(J) 6a 0.050 0.120 0.920 3.300 0.095 0.400 tr 0.003 0.160 0.409 570 2507a 0.044 0.110 0.560 3.000 0.080 0.465 tr 0.002 0.210 0.471 580 275 8a0.036 0.120 0.980 3.250 0.095 0.600 tr 0.004 0.220 0.612 575 260 9a0.030 0.130 1.100 3.100 0.110 0.720 tr 0.003 0.200 0.729 570 280

Examples 1 to 9 illustrate the combined effect of the content in termsof carbon and the highly carbide-producing elements for conventionalcontents of silicon. Examples 6a, 7a, 8a and 9a illustrate theparticular effect of silicon.

The effect on the weldability can be evaluated in particular using atest by means of which the minimum preheating temperature is determinedfor the auto-flange weld joint for which cracking is not seen to appearafter welding. This test involves producing weld seams with preheatingtemperatures of 150°, 125°, 100°, 75°, 50°, 25°, 5° and observing thejoints obtained in order to detect the presence or absence of cracks.

The results of this evaluation, which correspond to the examples oftables 1 and 2, are illustrated in the single FIGURE in which a firstlines 1 can be seen which represents the path of the minimumnon-cracking temperature as a function of the content of highlycarbide-producing elements for a silicon content in the order of 0.3%.

On this lines, it is found that, when the content of highlycarbide-producing elements increases and the content in terms of carbondecreases at the same time, up to a content of highly carbide-producingelements in the order of 0.6%, the minimum non-cracking temperaturedecreases. Beyond approximately 0.6%, the minimum non-crackingtemperature begins to increase again. Using micrographic examination inthe region of the cracks of the castings 9 and 10 which correspond tothe highest contents in terms of highly carbide-producing elements, itcan be established that the cracks appear in segregated zones whosehardness is found to be particularly high in spite of a relatively lowcarbon content. This high level of hardness of the segregated zonesprobably results from a co-segregation of the carbon and the highlycarbide-producing elements.

Upon examination of this line, it appears that an optimum level ofweldability is obtained for contents of highly carbide-producingelements of between approximately 0.4% and 0.65%.

Line 2, which corresponds to steels having compositions which arecomparable but which have a much lower silicon content than in theexample above, indicates that, when the content in terms of silicon isreduced, the minimum non-cracking temperature is reduced by 20° C. to25° C.

This effect of the silicon can be attributed to the effect of thesilicon on the thermal conductivity. By reducing the level of siliconwhich significantly impairs the thermal conductivity of the steel, thetemperature gradients are reduced in the heat affected zone, which hasthe effect of reducing the levels of stress.

This can be confirmed by measurements of the thermal conductivity of thecastings 6a to 9a and 6 to 9 which are comparable therewith.Measurements of this type would show that the thermal conductivity ofthe castings 6a and 9a is greater than that of the castings 6 to 9 byapproximately 10%.

With the steel according to the invention, it is possible to producecomponents for submarine hulls, for example, components which are cutfrom sheets having a thickness of between 40 and 60 mm, or forgedcomponents such as connection components whose great thicknesses can beup to 100 to 150 mm.

Using these components whose characteristics have been indicated above,it is possible to produce submarine hulls by assembling these componentsby means of welding on open air sites, the outside temperature beingable to reach 0° C. These components can be welded in a satisfactorymanner with no pre-heating, or with preheating to less than 25° C.

When the “coated electrode” method is used, which is generallyrecommended for the welding operations considered in this instance, theprecautions for use which are intended to limit the content of hydrogenintroduced must be complied with to the greatest possible extent, thatis to say, storage in the dry state and prior baking of the electrodes.The type of electrode used may correspond, for example, to the type E552NiMo in accordance with the standard EN757.

The solid wire MIG method which naturally introduces practically nohydrogen, is to be preferred wherever possible using, for example, awire of the type G55 Mn4Ni2Mo in accordance with the standard EN 12534.

These indications relating to the welding method constitute non-limitingrecommended values in this instance.

The invention claimed is:
 1. A process of producing a submarine hull,comprising (a) providing steel components, said steel components havingthicknesses of between 15 mm and 150 mm, said steel components having achemical composition consisting of, in weight percent:0.030%≦C≦0.080%0.040%≦Si≦0.19%0.1%≦Mn≦1.4%2%≦Ni≦4%Cr≦0.3%0.30%≦Mo+W/2+3(V+Nb/2+Ta/4)≦0.89%0.15%≦Mo≦0.89%W≦1.48%V+Nb/2+Ta/4≦0.004%Nb≦0.004%Cu≦0.45%Al≦0.1%Ti≦0.04%N≦0.0300% impurities resulting from the production operation, saidimpurities includingB≦0.0005%,P+S≦0.015%, the balance being iron, the chemical composition complyingwith the condition:410≦540×C^(0.25)+245[Mo+W/2+3(V+Nb/2+Ta/4)]^(0.030)≦460 the steelcomponents having a structure comprising: at least 90% of martensite, amaximum of 5% of residual austenite, and a maximum of 5% of ferrite, thesteel components having an elastic limit of between 480 MPa and 620 MPa(b) assembling and welding said steel components together to manufacturesaid submarine hull, wherein the steel components are welded by arcwelding using a coated electrode welding process or by arc welding usinga solid wire MIG welding process, wherein the welding is started withoutpreheating, including when the welding is carried out when ambienttemperature is less than 0° C., and the welding of said steel componentscomprises using filler metal.
 2. The process according to claim 1,wherein:0.1%≦Mn≦1%.
 3. The process according to claim 1, wherein:0.11%≦W≦1.48%.
 4. The process according to claim 1, wherein:2.5%≦Ni≦3.5%.
 5. The process according to claim 1, wherein:Cr≦0.15%.
 6. The process according to claim 5, wherein:Cr≦0.09%.
 7. The process according to claim 1, wherein:425≦540×c ^(0.25)+245[Mo+W/2+3(V+Nb/2+Ta/4)]^(0.30)≦450.
 8. The processaccording to claim 1, wherein:4%≧Ni≧2.7%.
 9. The process according to claim 1, wherein:0.15%≦Mo≦0.75%.
 10. The process according to claim 1, wherein:0.030%≦C≦0.055%.
 11. The process according to claim 1, wherein:0.030%≦C<0.060%0.040%≦Si<0.19%0.6%≦Mn<1.2%2.5%≦Ni<3.5%Cr<0.15%0.40%≦^(<)Mo+W/2+3(V+Nb/2+Ta/4)<0.59%0.15%≦Mo≦0.89%V+Nb/2+Ta/4≦0.004%Nb≦0.004%Cu<0.25%Al<0.04% titanium being absent or present in trace levels.